Malignant cells differ from nonmalignant cells in many ways, among them their ability to grow progressively and kill the host. We propose to investigate the genetic basis of this characteristic by means of somatic cell hydridization. When malignant cells are hybridized with diploid or other nonmalignant cells, the resultant hybrids are generally unable to grow progressively in a histocompatible host, i.e., malignancy is suppressed, until chromosome loss has occurred. The hypothesis we wish to test is that the key event in the re-expression of the malignant phenotype is loss of a specific chromosome (carrying a regulatory gene locus), or at most a very small number of such chromosomes derived from the nonmalignant parent. In spontaneous or experimental tumorigenesis, mutation or other hard-to-detect chromosomal change leading to loss of expression at the postulated regulatory locus may be a necessary event. In the hybrid cell system we propose to use, a more readily detectable whole chromosome loss could accomplish the same end and, in doing so, provide evidence of the role of a specific genetic change in the unregulated growth of malignant cells. Hybrids will be produced between malignant and nonmalignant cells in which specific chromosomes, or the entire complement of chromosomes, can be distinguished as to their parental origin. The karyotypes of malignant clones derived from the original hybrids by animal passage or growth in methylcellulose will be analyzed by chromosome banding methods. Selection of nonmalignant hybrid variants will be attempted, based on cell killing using differential heat sensitivity or differential growth in methylcellulose, and a chromosomal basis for the altered phenotype will be sought.